The Dutchman was leading Team Sky going into the stage from Sisteron to Vence, sitting in second place overall at 15 seconds behind race leader Luis Leon Sanchez (Astana).
Poels was tackling a descent in the final 7km of the race when the crash happened. He was quickly attended to, but was forced to abandon the race as a result of the incident.
Team Sky sport director Gabriel Rasch said: “The guys went down a pretty steep descent and the race was full on. I’m not sure what happened but Wout couldn’t make the corner and hit the barriers. He’s now at the hospital being checked over and our medical team is with him.”
The team said that it would release more information on Poels’s injuries later on Friday evening.
Poels’s race had been going well, with victory in the stage four individual time trial.
Sergio Henao (Team Sky) will now be sole leader for Team Sky at Paris-Nice. The Colombian won the race overall last year and is now positioned in eighth spot overall, 46 seconds behind Sanchez.
“Sergio looked good,” said Rasch. “He had to pay for some of the work he was doing in the last four kilometres but he looked strong. We’ll go all in for him now.”
Just two stages remain in the 2018 race: Saturday’s big climbing stage from Nice to Valdeblore La Colmiane, and the tricky final day starting and finishing in Nice.